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 91 
 on: February 13, 2019, 07:30:24 pm 
Started by Scott_Smith - Last post by David Satz
I've been in the anechoic chamber at Neumann in Berlin--but that isn't where they build their mikes any more.

At any rate, while a 1-meter effective measuring distance is a somewhat common practice, it isn't specified by any established standard. As I understand the IEC standard, frequency response curves shouldn't include any proximity effect at all. (At one meter there is some proximity effect for cardioids, more for super- and hypercardioids, and even more for figure-8 microphones--see the attached excerpt of an old Gotham Audio publication on the U 67, for example.)

If an anechoic chamber is used--as Kai mentions, there are alternatives--measurements are generally carried out at a somewhat greater distance, then "corrected" so that the published curves will show the low-frequency response that would be expected at the chosen distance, whatever it is. Even that is a generalization, since the geometry of the sound source--point versus plane or in between--is a major variable as well.

That "chosen distance" has been 1 meter for several of the most respected manufacturers, including Schoeps and Neumann, for a long time. For other manufacturers (DPA among them) it is less, e.g. 30 cm. There also are manufacturers who choose the measurement distance on a product-by-product basis according to the intended use, e.g. if a microphone is designed for close speech pickup it will be measured at (or "corrected to") a smaller distance than if it is intended for general studio use.

Of course when you "correct" to (or actually measure from) a point that's closer than another manufacturer uses, you raise the apparent bass response in your curves relative to theirs. Most manufacturers don't say what their measurement distance is--and even if they did, it's not exactly obvious how to use that information when interpreting the low end of two published response curves if they were made at different effective distances. This makes it difficult to compare the low-frequency response of directional microphones between different manufacturers, which I find unfortunate.

--best regards

 92 
 on: February 13, 2019, 05:52:12 pm 
Started by Scott_Smith - Last post by Kai
I can't comment on what Neumann currently does, but I have first-hand information from Schoeps, being located in my hometown.
They do not use an anechoic chamber, but a long tube to generate a clearly defined sound field.

Unechoic chambers have certain disadvantages:
- There is a lower limiting frequency connected to the chambers size and kind of damping.
- They are not perfectly free from reflections, for example you need something to walk on which is usually done with steel net, and the damping materials cannot be made 100% unreflective too.

There are other techniques for microphone measurements that can exclude reflected sound.
One of the most popular is named Time Domain Spectrometry, TDS. 
The idea behind these types of measurements is, that reflected sound takes longer to reach the device under test than the direct sound stimulus.
So reflections can be excluded by using a very narrow band sweeping filter that has swept out of their way at the time the reflection arrives.
The stimulus would be a swept sine wave in this case, as usual.

I have used TDS with great success in absolutely normal rooms, and the other advantage is, the room does not even need to be very quiet, as noise is filtered out by some amount too.

Cardiod mic's frequency response measurements are usually referenced to 1m, except for special types that are made for close up use only.
These measurements only show a small window into the real situation, as going closer to the source will boost the lower frequencies up to 20dB due to the proximity effect, and going further away will leave a roll off in the range below ca. 100Hz.
The rolloff is partly compensated by room reflections in real recording situations.

BTW, LDC (Large Diaphrag Condenser) cardioid mics are not better in this regard, just different.
Usually their proximity effect is less pronounced, as they tend to go into omni characteristics for lower frequencies.

This all is purely academic, finally your ears needs to judge the result.
I would go so far to say that studio microphones all have quite similar published measurements and sound much more different than the measurements would suggest, on the other hand.

 93 
 on: February 13, 2019, 03:56:19 pm 
Started by Scott_Smith - Last post by klaus
I asked David Josephson to chime in. He is the expert on measuring protocols for condenser microphones.

 94 
 on: February 13, 2019, 03:51:57 pm 
Started by mikezietsman - Last post by klaus
I guess you misinterpreted my sentence, clumsily written:
("Keep the M269 head in cardioid when testing on the U67 body. That's the only pattern that will work in this combination."
I was referring to the ONLY combination that will work without modifying mics or heads: M269 head on U67 body, and it will only work in cardioid.

 95 
 on: February 13, 2019, 02:25:29 pm 
Started by Scott_Smith - Last post by Scott_Smith
Klaus:
I've searched for this information elsewhere, bu am not finding anything definitive. Can you enlighten me as to whether Neumann used a standard distance from speaker source to mic when performing response plots and polar plots? I have seen the distance of 1 meter bandied about, but don't know if that is in fact correct. Has the distance varied over time? Is it the same for all microphone types?

Also, are the polar plots typically conducted with the diaphragm located at the 0 degree axis point, or with the front of the mic grille located at 0 degrees?

I would also be interesting to know what the termination impedance is of the mic preamps typically used. (I am assuming about 1.2K ohm).

S. Smith   

 96 
 on: February 13, 2019, 11:56:55 am 
Started by mikezietsman - Last post by mikezietsman
Just a quick follow up...

It would appear that a u67 head does not work on an m269 in cardioid mode. Luckily it also appears to have not done any damage. (there was just no output)

Thank you so much to everyone for their advise on the power variations...

My current strategy until I can get something like Uwe's reccomendation, is to be powered down before the blackouts (we have a nationwide schedule) and to make sure I stay powered down for a good ten minutes after the power comes back on.

 97 
 on: February 13, 2019, 11:24:14 am 
Started by mikezietsman - Last post by Jim Williams
Bourns makes the MOV devices, available from Mouser and other distributors. The MOV-07DxxxK series is small and can be soldered across the IEC AC power inlet jack. You pick the surge voltage from 18 to 820 volts. Use the 120 volt for the USA, the 250 volt for the EU.
p/n 652-MOV-07D121K for 120 volts, p/n 652-MOV-07D101K for 250 volts. Those are Mouser part numbers. They only cost 10 cents each in single quantities, a rather low cost "insurance" policy.
www.mouser.com

 98 
 on: February 12, 2019, 03:06:32 pm 
Started by mikezietsman - Last post by klaus
Jim,
Please expand on your post in a non-technical language everyone can follow.
Thanks,
KH

 99 
 on: February 12, 2019, 11:46:45 am 
Started by mikezietsman - Last post by Jim Williams
One can fit a MOV device (metal oxide varister) across the AC pins on the internals of the gear. Those clamp surge voltages and are effective and cheap.

 100 
 on: February 11, 2019, 05:40:20 pm 
Started by mikezietsman - Last post by Kai
Will voltage dips in the mains supply pose any risk to the ac701?

We’re currently in a phase of scheduled blackouts and I suspect the supply is not great.
A power line situation like this is always connected to large overvoltage spikes which are likely to kill all kinds of electronic devices.
Even here in Germany, when I had to put an over-voltage protection into my central breaker box, failure of studio devices immediately was reduced from often to very few.

Here is an example of such a protection:
https://www.hager.de/modulargeraete/ueberspannungsschutz/ueberspannungsableiter-typ-3/spn203n/950999.htm

These protections are simple, cheap ($200 for a professional version), very effective, and pay off soon. The stationary breaker-box versions are much more effective than the ones built into multi-outlet-strips.
Installation must be done by a qualified electrician.

Over-voltage protections don't deal with smaller scale voltage variations, they cut surges of 1250+ Volts only.
For sensitive devices I suggest to ADDITIONALLY use a battery backup computer UPS (uninterrupted power supply - K.H.) with integrated regulation and sine-wave output.
The cheap ones don't regulate and put out waveforms with huge amounts of overtones (called power line voltage distortion in %) which creep into the audio, so get information before you buy.

A good UPS can bridge blackouts for several minutes, and give you the possibility to properly power down your computers without data loss, too.

And while we're at this: I strongly recommend to install a "ground fault circuit interrupter", other names see:
https://en.m.wikipedia.org/wiki/Residual-current_device

This thing saves lives, especially with historical music equipment not built by today's safety standards.
In my studio it has saved at least one person from beeing electrocuted, who had the great idea to solder a directly connected line level audio output to the internals of a TV set he used as monitor for his computer.

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